Within the field of oilwell drilling, it has been known to use powered hydraulic wrenches or tongs to fasten and unfasten threaded sections of drillstring pipe during the process of removal or reinsertion of drillpipe into a bore hole for drilling. The usual practice involves operations on the working platform of an oilwell rig or a drilling rig in which a continuous pipestring or drillstring made up of threaded segments of pipe extends down through a bore hole and is rotated to drive a drill bit. The individual segments of drillpipe are hollow, and provide a passageway for a drilling fluid, usually a viscous water or oil based fluid known as drilling mud, which both lubricates and aids the cutting of a drill bit and flushes the cut particles from the bore hole.
The entire drillstring is rotated for drilling by means of a powered rotary table and an associated tool, known as a kelly, for imparting the rotary motion of the table to the drillstring. When it is necessary to remove the drillstring from the borehole a block and tackle is used to raise the kelly, removing the drillstring from the borehole a distance sufficient to expose at least one threaded section of pipe of which the drillstring is made up.
The drillstring is broken down by unthreading or unscrewing these pipe segments, normally by the use of a pair of powered rotary hydraulic wrenches of the type known as hydraulic tongs. In this usage, the tongs, which, because of the torques involved, are relatively large, heavy articles, are suspended by a counterbalancing mechanism within the drill rig adjacent to the rotary table.
A first non-rotating tong called the backup tong, is clamped below a threaded joint onto a section of the drillstring; it serves, in combination with a set of inserted slips within the rotary table which prevent the drillstring from falling into the borehole, to secure a lower section below a threaded joint against rotation. A second rotary power tong is clamped to a section of the drillstring above a threaded joint. The torque applied by the rotary power tongs against the fixed resistance of the lower fixed tong provides adequate force across the threaded joint to unscrew the joint, releasing the upper drillstring segment or pipe section for removal.
After an oilwell has been drilled and placed into production, the produced fluids, under pressure, are transported up a pipe section known as production tubing. The primary physical difference between a string of production tubing and a drillstring is that the production tubing is physically smaller, and is fixed within the drillbore rather than being rotating. Further, production tubing is only removed in the event of a downhole problem requiring access to the formation to increase production or because of a downhole blockage. In this environment, a major problem occurs when the tubing section becomes jammed. There are a number of events which can cause such blocking; but the most common occur when a wireline tool, which is periodically lowered within the tubing to test the downhole formation, for some reason becomes jammed, and when a produced fluid which will usually include sand and particles, for any of a number of reasons, cakes up and forms a solid plug within the tubing.
When this production tubing is pulled it will commonly be found that the length of time that it has remained downhole causes chemical changes and corrosion to the threaded joints of each of the tubing sections and that significant torque is required to break the tubing apart.
Since downhole pressures can be extremely high in downhole production operations, and since it is typical to encounter various gasses entrapped within downhole fluids, such a blockage can create a dangerous situation within the production tubing. Some sections of the tubing between blockages can contain trapped fluids and gasses at relatively high pressures. When the tubing is uncoupled as described above these pressures react against the threaded joint, causing a sudden rupture of the joint as it is unscrewed and a dangerous expulsion of liquids and particles at a level of the drill floor which is occupied by working personnel. In addition, the sudden release of pressure can propel the upper tubing section, which is restrained only by the draw works, within the rig creating a special hazard to personnel and equipment.
Even where no pressure release is involved, the existence of trapped sections within a drillstring can result in the loss of significant amounts of valuable drilling fluids. The drillstring is broken apart above the drill floor, well removed from the normal provisions for capturing the flow of drilling fluid and recovering the fluids. Some drilling fluids such as calcium bromide are extremely expensive and represent a significant monetary value if lost or contaminated.
In the current art there is no known reliable method for preventing the sudden release of entrapped gasses or the loss of valuable trapped fluids within a pipestring containing internal blockage.